The present invention relates to the field of installing and implementing rigid steel pipes under the sea from a floating support or vessel on the surface and going down to equipment that is immersed, preferably down to the sea bottom. Such rigid tubes may be so-called “service” tubes that are implemented for testing or maintaining said undersea equipment from the surface of the sea, or indeed pipes for transporting production or service fluids to such equipment, in particular undersea pipes for transporting petroleum or associated fluids, or wellheads or other pieces of equipment. Such rigid tubes are intended more particularly for testing such undersea equipment from the surface by conveying liquids or gases thereto at varying temperatures and pressures. In particular, the tests implemented consist in filling the production undersea pipe with a liquid in order to clean the line, e.g. treated sea water under pressure, and causing a scraper to pass therealong in order to clean it, for example. The undersea pipe may subsequently be dried by delivering a gas such as air thereto. The production pipe may also be subjected to an immersing method using mono-ethanol-glycol and/or nitrogen.
Service tubes are rigid tubes made of steel or metal or of any other material, in particular composite material, that are wound on a drum at the surface and then employed in immersion in the sea in order to be connected to equipment that is immersed or at the sea bottom, and then perform a said test or maintenance operation by sending down liquids or gases, and they are finally recovered from the vessel or floating support by winding. Such windable service tubes are also known as “coiled tubing” and may be unwound and rewound several times. In general, their top ends remain wound in part on the drum so such a tube is not completely unwound. Nevertheless, in certain circumstances, the top end of the tube may be completely unwound and secured at the surface.
In practice, such service tubes present diameters of relatively small size compared with the diameters of standard undersea oil production pipes, and in particular they may be steel service tubes with diameters of less than 10 inches (″), and more particularly in the range 1.5″ to 6″, and still more particularly in the range 1.75″ to 4.5″, more particularly 50 millimeters (mm) to 100 mm, for taking action at depths of more than 1000 meters (m) or indeed more than 2000 m.
On being wound, such rigid steel service tubes are subjected to deformation that is “plastic” in the mechanical meaning of the term, i.e. the stresses that are applied to the tube go beyond the elastic limit of the tube and it is thus permanently deformed. Thereafter, while the tube is being unwound, the tension that is applied in order to unwind it serves to straighten it out on leaving the drum, possibly in combination with a straightener. More particularly, rigid steel service tubes of the type in question present elastic limits in the range 335 megapascals (MPa) to 750 MPa. Rigid service tubes of this type are described in the prior art, in particular in WO 2012/051335.
Because of successive unwinding and winding, and also because of movements of a service tube while it is being deployed and in operation, a service tube is subjected to high levels of localized stress at the point from which it is suspended at the surface. Thus, the rolling, pitching, and heaving of the floating support or vessel, and also the action of waves, wind, and/or currents on the service tubes and on the floating support or vessel give rise to high levels of bending at the point from which the tube is suspended and/or fastened to the floating support or vessel, with this being particularly severe when the length and thus the weight and also the pressure of the fluid conveyed in the service tube are all large.
In order to mitigate that problem, the practical solution that is presently used consists in unwinding additional length of tube on a regular basis, in particular lengths of a few meters, so as to shift the zone of the service tube on which the stress forces act. Nevertheless, that solution is applicable only to service tubes of small diameter, and in particular of diameter less than 50 mm, and for service tube operations of duration shorter than one day, suitable for use when taking action at depths limited to less than 1000 m. For periods that are longer, and for taking action at depths that are greater than 2000 m in particular, and also for tubes of diameters that are larger, the rigid tube becomes excessively fatigued and runs the risk of breaking. More particularly, at present, operations continue for a few hours and the rigid line can be unwound by a few meters after each operation in order to minimize the fatigue that occurs in the same zone of the rigid tube, whereas operations at great depth continue for more than one day or indeed more than one month, and the risk of the tube rupturing due to fatigue becomes very great, and in the event of the tube accidentally rupturing there can be consequences that are disastrous for the equipment, for personnel, and for equipment on the sea bottom.
That solution is therefore not applicable for use at great depths where the service tube continues to be used for weeks and where the diameters needed are greater than 50 mm.
Another problem that is encountered relates to connecting the termination of the rigid tube to test equipment or to equipment that is to be tested or operated. The rigid metal tube needs to present a connection element at its end, in particular an element of the automatic connector type, which equipment is of diameter that is greater than the diameter of the service tube in order to enable its end to be connected to a semi-rigid pipe or more usually to an intermediate flexible pipe. Unfortunately, it is not easy to assemble such a connection element to the tube once its end is immersed at the sea bottom. One presently-known solution consists in assembling said connection elements at the surface to the end of the service tube before deploying it to the sea bottom.
Bend stiffener type devices or bend restrictor type curvature limiters are known that are applied to the ends of flexible pipes, as described in EP 2 503 093, FR 2 952 118, and FR 2 871 511. Bend limiters or stiffeners for flexible pipes are generally made in the form of conical parts made out of synthetic materials, in particular polyurethane type elastomer material. Parts of this type that are made of steel are also known, and they are applied to the end of a rigid steel pipe of “riser” type in order to embed it in a part for providing a transition in second moment of area (or “inertia”) of the type known as a “taper” joint or as an “adaptor” joint, as described in WO 2009/138610. Such parts extend the existing pipe, and as a general rule they are welded to the pipe or they are assembled thereto by means of a flange.
The conical shape provides a transition in second moment of area by progressively and continuously reducing in diameter starting from the point that suffers the greatest mechanical stress. Because such conical parts are secured of the end of the pipe, the forces to which said pipe end are subjected are transferred to the conical part and the increase in its section enables the overall stress to be spread out, thereby providing a smoother stiffness transition and thus a reduction in local stresses. The section of the conical part decreases progressively as a function of the decrease in said stress, with the stress being at its maximum at the point where the end of the pipe is connected or suspended.
Solutions of that type as applied to a tube as described above are inappropriate, given the need to allow the top end of the rigid tube to be wound out and wound in at its point of suspension at the surface.